In electrophotographic copying processes, an electrostatic latent image is formed on a photoconductive surface and then developed with a thermoplastic toner powder. The toner is thereafter fused to a substrate by passing the substrate, (e.g., a sheet of paper on which toner powder is distributed in an imagewise pattern) through the nip of a pair of rolls. At least one of the rolls is heated and has a resilient surface so that the thermoplastic toner powder softens and bonds to the substrate.
A persistent problem in this operation is the adherence of heated toner to the fusing roll. As a result, a false offset image appears on sheets of paper subsequently passing through the rolls and performance of the fusing roll degrades. Upon continued heating, such roll degradation causes uneven surfaces and defective patterns in thermally fixed images.
Toner fusing rolls are composed of a cylindrical core which may contain a heat source in its interior and a resilient covering layer surrounding the core. A thin layer of a suitable primer is advantageously coated on the surface of the core in order to improve bonding to the covering layer. Roll coverings are commonly made of fluorocarbon polymers or silicone polymers, such as poly(dimethylsiloxane) polymers, which have a low surface energy to minimize toner adherence. Frequently, release oils (e.g., containing functionalized poly(dimethylsiloxanes)) are applied to the roll surface to prevent adherence of toner to the roll. However, such release oils can interact with the roll covering layer to cause eventual roll swelling, softening and degradation.
U.S. Pat. No. 4,807,341 discloses fusing rolls comprising a crosslinked elastomer prepared by copolymerization of dimethylsiloxane, diphenylsiloxane and vinylsiloxane monomers. Incorporation of 5-15 mole percent of diphenylsiloxane recurring units afforded improved resistance to swelling by release oils.
U.S. Pat. No. 4,659,621 discloses a roll for use in electrostatography with a surface layer comprising a crosslinked elastomer prepared by copolymerization of a polyfunctional silicone hydride, a vinyl polyorganosiloxane and a polyorganosiloxane filler. The roll afforded improved resistance to swelling by release oils.
Roll coverings usually contain a filler to strengthen the polymeric covering layer. A particularly useful filler is thermally conductive and provides thermal stability to the polymer layer. Such fillers are needed when fabricating fusing rolls with the commonly used poly(dimethylsiloxane) rubber polymers.
Fusing rolls with covering layers made of poly(dimethylsiloxane), despite their common use, have a number of deficiencies. There thus exists a need in the art for a polymeric covering layer which is thermally stable, resistant to chemical degradation, and thermally conductive.